1. Field of the Invention
This invention relates to a method of enhancing the bond of an orthodontic appliance that is directly bonded to a tooth surface, and is particularly useful for enhancing the bond of an orthodontic appliance made of a plastic material.
2. Description of the Related Art
Orthodontic treatment concerns movement of malpositioned teeth to orthodontically correct positions. In certain types of orthodontic treatment, tiny, slotted appliances known as brackets are affixed to anterior, bicuspid and cuspid teeth, and other appliances known as buccal tubes are secured to the molar teeth. An orthodontic archwire is placed in the slots of the brackets and is tied in place using small elastomeric O-rings or metallic ligature wires. Ends of the archwire are inserted into channels of the buccal tubes.
As orthodontic treatment progresses, the archwire serves as a track to guide movement of the teeth toward desired positions. Bends, loops or twists are often placed in the archwire to urge movement of the teeth. Alternatively, one or more elastic members secured to brackets or buccal tubes of certain teeth provide a force to move the associated teeth to their correct positions.
In the past, it was common practice to weld each bracket to a metal orthodontic band that was, in turn, placed around the selected tooth in encircling relation and cemented in place. Such bracket and band assemblies provided a strong connection to the underlying tooth but were also highly visible and often an embarrassment to the patient. The use of welded bracket and band assemblies was also troublesome in that teeth vary widely in size and shape and a certain amount of time is typically required to carefully select each band so that the chosen band complementally fits onto the corresponding tooth.
In more recent years and with the development of new adhesives, the use of brackets that are directly bonded to teeth has become increasingly popular. For example, it is now common practice to bond metallic and ceramic brackets directly to the tooth surface. Bonding the bracket to the tooth surface may be carried out by chemical adhesion, or a combination of chemical and mechanical adhesion, and is a function of the type of adhesive used as well as the material of which the bracket is made.
Regardless of the type of adhesive and bracket used, it is important that the bracket is bonded to the tooth with sufficient strength so that the bracket does not spontaneously debond from the tooth under normal conditions as orthodontic treatment progresses. If a bracket does debond from the tooth before treatment is finished, the patient should return to the orthodontist so that the archwire can be removed and a new bracket bonded in its place. Obviously, such a procedure is time consuming and a nuisance both to the orthodontist as well as to the patient.
The manufacturers of orthodontic appliances have directed attention in recent years toward a variety of manufacturing methods for enhancing the mechanical bond between the base of the bracket and the adhesive. Some brackets, for example, are provided with a dimpled surface to increase the surface area of contact between the bracket and the adhesive, such as is shown in U.S. Pat. No. 4,243,386. Others have suggested the use of small particles, roughened surfaces or other means to provide increased surface area, such as described in U.S. Pat. Nos. 4,626,209 and 4,752,221.
A particularly effective method of enhancing the mechanical bond strength of a directly bonded orthodontic appliance is by providing a bonding base for the appliance with overhanging portions, so that one or more undercut regions are attained. When such an appliance is bonded to a tooth surface, the adhesive flows into the undercut regions and hardens, establishing a mechanical interlock between the adhesive and the bonding base.
Metal brackets sold by 3M Unitek Corporation under the trademark "DYNA-LOK" are made in a machining operation, and during machining a series of slots are made in the base. Edges of the slots are subsequently deformed by a knurling process such that the slots present undercut channels. A drawing of one type of "DYNA-LOK" brand bracket is shown in U.S. Design Pat. No. 290,040.
Other metal brackets, such as 3M Unitek's miniature twin brackets, are manufactured using a metal injection molding process. A mold assembly has cavities that provide upstanding pegs or projections on the base of the bracket. The brackets are tumbled during a finishing operation in a tumbling mill using a media that peens over the outer edges of the projections. An example of such peened-over projections is shown in, for example, U.S. Design Pat. No. 340,523.
Other methods for providing undercut regions include the use of a porous wire mesh that is fixed to the bracket base, such as is illustrated in U.S. Pat. No. 4,068,379. In addition, U.S. Pat. No. 4,110,678 describes the use of a sandblasting and oxidizing solution to provide undercut pores in the base of metal brackets.
Another method for making orthodontic appliances with a bonding base having undercut regions is described in U.S. Pat. No. 5,393,486. The method described in this reference involves in one embodiment the use of a plastic mesh that is embedded in the base of the bracket during a molding operation, and is subsequently degraded by heat, solvent or other process to establish a witness impression of undercut regions.
Many attempts have been made over the years to use orthodontic brackets made of plastic materials rather than metal because brackets made of plastic material are often less visible in the oral cavity and hence more aesthetic than brackets made of metal. However, conventional plastic brackets are often considered unsatisfactory because chemical adhesion of plastic brackets directly to the teeth is often not sufficiently strong to prevent premature, spontaneous debonding of the bracket when certain conventional adhesives are used. Moreover, the machining and/or tumbling processes described above in connection with metal brackets for facilitating a mechanical interlock are not generally satisfactory for one reason or another when attempted for plastic brackets.
As can be appreciated, there is a continuing need in the art for new efficient and effective methods of manufacturing plastic brackets and for enhancing the bond of plastic brackets to teeth. Preferably, such methods would also be useful when used in connection with appliances made of other materials that exhibit thermoplastic characteristics during at least one phase of manufacture.